1. Field of the Invention
The present invention relates to the art of metal alloy injection molding and, in particular, to methods and apparatus for removing contaminants from parts used in the injection process.
2. Related Prior Art
In the typical metal alloy injection molding process an extruder screw is used to move the alloy toward a collection site and force the alloy into a mold. Occasionally it is necessary to remove the screw from the injection unit. When this is done it is difficult to ensure that some of the metal alloy does not adhere to the screw. This adhering alloy must be removed from the screw before it is put back in the injection unit. Although the common practice in removing screws is to first purge the metal alloy from the barrel, the screws that are removed typically are encased in a certain amount of alloy, particularly in the check valve region. Some of this alloy can be chipped away with a brass chisel, however a significant amount of alloy remains on the screw. In order to remove this remaining alloy, the relatively high solubility of the metal alloys typically used in injection molding, such as magnesium, aluminum or zinc based alloys, in acidic media has been exploited. For example, magnesium based alloy was removed by dissolving it in hydrochloric acid. This process involved soaking the screw in a bath of hydrochloric acid. However, in those instances where the screw is coated with a corrosion prevention coating or layer such as Stellite, the hydrochloric acid can effect the interface between the coating and the screw. A phosphoric acid solution was used in place of the hydrochloric acid. While the phosphoric acid did not interfere with the coating-screw interface, other problems still existed with the process.
As illustrated in FIG. 1, the prior art process involves placing the screw in a tubular leach vessel 1. The vessel 1 is filled with an aqueous phosphoric acid bath 2 that circulates through the leach vessel 1, passing over the screw placed in the bath 2 and exiting to an acid holding tank 3. The leach vessel 1 consists of a PVC pipe with end caps 4 to seal the vessel 1. Polypropylene tubes allow the acid to enter and exit the vessel. Under acidic conditions, an electrochemical reaction occurs at the metal alloy-acid interface resulting in the oxidation of the metal alloy compounds to form soluble ions or ionic complexes. Equation (1) indicates the reaction process for magnesium. Similar equations can be given for zinc and aluminum alloys. The oxidation of the magnesium or other metallic compounds is accompanied by a reduction in the hydronium ions resulting in the evolution of hydrogen, as indicated in equation (2). The oxidation-reduction reactions are exothermic, meaning the reactions proceed spontaneously and liberate energy. The overall generic equation is given by (3).                               Mg          0                ⁢                  →                                    xe2x80x83                        ⁢                          acid              ,              water                        ⁢                          xe2x80x83                                      ⁢                              Mg                          +              2                                +                      2            ⁢                          e              -                                                          (        1        )                                                      2            ⁢                          H              2                        ⁢                          O              +                                +                      2            ⁢                          e              -                                      ⁢                  →                                    xe2x80x83                        ⁢            water            ⁢                          xe2x80x83                                      ⁢                              2            ⁢                          H              2                        ⁢            O                    +                      2            ⁢                                          H                2                            ↑                                                          (        2        )                                                      {            Metal            }                    +                      {            Acid            }                          →                              {                          xe2x80x83                        ⁢                                                                                soluble                    ⁢                                          xe2x80x83                                        ⁢                    metal                                                                                                                    ions                    ⁢                                          /                                        ⁢                    ionic                    ⁢                                          xe2x80x83                                        ⁢                    complexes                                                                        ⁢                          xe2x80x83                        }                    +                      {            hydrogen            }                    +                      {            energy            }                                              (        3        )            
A typical bath consists of 25 wt % (10.5%) phosphoric acid. Catastrophic failure of the cleaning apparatus can occur if the process is not properly controlled. The system illustrated in FIG. 1 fails to provide adequate controls. First, the system is closed. There is no mechanism for the removal of hydrogen produced in the process. Explosive conditions can occur when hydrogen is allowed to accumulate in a confined space. Further, pressure can build up in a closed system. Second, there is no means of managing the heat of reaction associated with the dissolution of the metal in acid. Management of the heat of reaction will provide control of the rate of reaction and prevent xe2x80x9crun-awayxe2x80x9d reactions from occurring. Also, removal of the heat of reaction will prevent other reactions that can occur at higher temperatures from taking place. Third, the cleaning apparatus does not ensure controlled failure as no pressure relief valves are used to manage pressure build up in the leach vessel.
It is evident that the prior art cleaning apparatus does not effectively manage the chemical reactions associated with the dissolution of metal alloys in acidic media. In order to ensure safe operation, the cleaning apparatus should be designed as a chemical reactor, giving consideration to the chemical kinetics and thermodynamics of the processes involved.
The primary objective of the invention is to provide a method and apparatus for safely and efficiently removing metallic alloy contaminants from a metallic substrate.
Another object of the invention is to provide a method and apparatus for safely and efficiently removing magnesium based alloy from a metallic substrate such as stainless steel or tool steel or super alloys.
The invention provides a method and apparatus for removing contaminants from the surface of a metallic article where the article is placed in a continuously flowing aqueous bath to which is controllably added acid for dissolving the contaminants and cleaning the surface of the article. A purging gas flows over the surface of the bath so that the build-up of hazardous conditions within the bath can be avoided.